Mammut pacificus sp. nov., a newly recognized species of mastodon from the Pleistocene of western North America

Diagnosis

A species of Mammut differing from Mammut americanum in the following characteristics: M3/m3 significantly narrower relative to length; six fused sacral vertebrae in later ontogenetic stages (usually five in M. americanum, with a range of four to six); femur with a greater midshaft diameter relative to length; absence of mandibular tusks and associated alveoli (variably present in M. americanum); smaller basal diameter of tusks in males for a given age.

Cranium

The cranium of WSC 18743 (Fig. 1; M37481) is largely complete. There is some dorsoventral crushing that slightly flattened the dorsal side of the braincase; this crushing does not appear to have affected the ventral half of the cranium. The alveoli for the tusks appear to be crushed as well, particularly on the left side.

In general morphology, the cranium does not differ significantly from that of Mammut americanum, when allowing for individual, sexual, and ontogenetic variability. WSC 18743 falls into Laws group XXII (39 ± 2 AEY), which is ontogenetically somewhat older than AMNH 9951 (the “Warren mastodon”) from Newburgh, New York (based on our images of this specimen and the written description in Warren (1852)). The frontals of WSC 18743 more fully cover the temporal fossae in dorsal view than does AMNH 9951, but this may be affected by the dorsoventral crushing in WSC 18743. In any case, this feature exhibits some ontogenetic variability (compare, e.g., AMNH 9951 and AMNH 14535 in Osborn (1936)). Although WSC 18743 and AMNH 9951 both represent male mastodons and are of roughly similar ontogenetic ages, the cranium of WSC 18743 is considerably shorter, with a length of one m (measured along the midline on the dorsal surface) (Table 1). According to Warren (1852), the cranial length of AMNH 9951 is 48 inches (1.23 m).

Osborn (1936) noted that M. americanum specimens differed from each other in the position of the tusk alveoli relative to the maxillary tooth rows, a feature that he considered to be sexually dimorphic and related to the larger size of the tusk in the male. In AMNH 14292 (approximately LG XXI), which Osborn (1936) considered a female, and AMNH 17727 (LG V or VI), considered a juvenile male by Osborn (1936), there is a vertical step between the maxillary tooth row and the ventral margin of the premaxilla that forms the tusk alveolus when seen in lateral view (e.g., see Osborn, 1936, Figs. 131 and 132). In contrast, in AMNH 9951 (LG XX or XXI), YPM 12600 (LG XIX or XX), and AMNH 14535 (LG XVI or XVII), all regarded by Osborn (1936) as males, the ventral margin of the premaxilla is only slightly raised above the tooth row, with no distinct step. Two specimens, ISM 71.3.261 (the “Buesching mastodon,” LG XIX) and a UMMP unnumbered specimen from Michigan (the “Fowler Center mastodon,” LG XVI or XVII) are interesting intermediate stages, with a small but distinct step. In IPFW 1 (the “Routsong mastodon” LG XV), an apparent female, the premaxilla is elevated well above the tooth row, but instead of a sharp vertical step there is a more gradual transition from the tooth row to the premaxilla, showing that there is some variability in this feature.

In WSC 18743 there is a distinct vertical step between the premaxilla and the maxillary tooth row (Fig. 1C). Even though this is a mature specimen (LG XXII), and interpreted as a male based on pelvic measurements, this condition is more similar to M. americanum specimens considered by Osborn (1936) to be females or juvenile males. As tusks continue to grow late in mastodon ontogeny, it might be expected that in males there would be some ontogenetic variability in the expression of this character. Indeed, referred specimen WSC 8817, which has an estimated age of 55 ± 4 AEY (LG XXVIII), has tusk alveoli that extend ventrally to just above the tooth row (Fig. 7). This suggests that M. pacificus may have reached its maximum tusk size later in ontogeny than M. americanum, although a full analysis of this possibility will require a detailed examination of tusk diameters, premaxilla morphology, and ontogenetic ages of both M. pacificus and M. americanum.

The left and right maxillary tooth rows in WSC 18743 and in referred specimens WSC 10829 (a possible male, based on tusk size) and WSC 8917 (a possible female, based on tusk size) are parallel or nearly so (Figs. 1B, 11 and 18C). This is in contrast to some other specimens referred to this new species, including WSC 22587 (Fig. 10), UCMP 114599 (Fig. 12), and UCMP 36684 (Fig. 14), in which the tooth rows are posteriorly convergent. Osborn (1936) noted that similar variability in M. americanum does not appear to correlate with sex or ontogeny. M. pacificus seems to exhibit similar variability, with both morphologies found across several ontogenetic ages, and at least the parallel tooth row form found in both males and females.

Mandible

The mandible of WSC 18743 (Fig. 2) is relatively elongate, with a smoothly downturned anterodorsal surface of the mandibular symphysis as is typical of Mammut. The ascending ramus is angled slightly posteriorly, with the anterior margin forming an angle of about 110° with the tooth row. The anterior margin of the ascending ramus extends vertically to from a somewhat recurved coronoid process. Posteriorly the ascending ramus expands transversely into the mandibular condyle, the dorsal margin of which is slightly higher than the apex of the coronoid process. There is a partially resorbed alveolus for the m1 on the right side only. There are no lower tusks, and no trace of alveoli for them. There are two pathologies on the right dentary, a bony growth near the anterior tip of uncertain origin and a deep groove with swollen margins on the lateral surface adjacent to the anterior end of m2. The more posterior pathology appears to have resulted from an impact injury. Fisher (2009) proposed that male specimens of M. americanum show frequent injuries that resulted from intraspecies combat, which is one possible source of the pathologies observed in WSC 18743. The entire mandible is asymmetrical, with the right tooth row shifted posteriorly relative to the left. Given that the degree of asymmetry is variable across the length of the dentary, it is interpreted as a response to the trauma suffered by the right dentary, rather than due to taphonomic processes.

There is some variability in the shape of the ascending ramus among different specimens that has no direct correlation with age or body size. WSC 19730 (LG XIX), WSC 8817 (LG XXIX), LACM-P23 26389 (LG III), LACM-HC 475 (LG II or II), and UCMP RSF 0201 (LG I or II) have posteriorly inclined ascending rami similar to WSC 18743, while in LACM 152669 (LG XXII), LACM 128927 (LG XII), LACM-HC 87073 (LG XXII), SBMNH-VP-3341 (LG XX), and SBMNH-VP-3342 (LG XXII), the anterior margins of the ascending rami are nearly vertical. There is also discernible variability in the angle between the dentaries in dorsal view, from nearly parallel to posteriorly divergent (Fig. 19). This divergence is most noticeable in the portion of the dentaries posterior to the tooth row and may become less pronounced with age as the mandible increases in length (compare, e.g., Figs. 19A and 19M). However, this character may be subject to subtle taphonomic alteration, especially if the mandible is lying on its side when buried.

Thirteen specimens of M. pacificus preserve the mandibular symphysis, and range from LG I to LG XXIX (WSC 18743, WSC 19730, WSC 8817, LACM 128927, LACM-HC 38, LACM-HC 87073, LACM-HC 475, LACM-HC 1631, LACM-P23 26389, UCMP 86140, UCMP 212944, UCMP RSF 0201, and SBMNH-VP-3342) (Fig. 19); all of these specimens lack mandibular tusks or any trace of tusk alveoli. Moreover, at the two California localities that have produced numerous mastodon remains (Rancho La Brea and Diamond Valley Lake), no isolated elements consistent with mandibular tusks have been recovered. This is a remarkably stable pattern when compared to M. americanum. While the geographic, temporal, sexual, and ontogenetic distribution of mandibular tusks in M. americanum is not fully understood (Laub, 1999, 2009), there does appear to be a trend toward reduction of the mandibular tusks over time. Every specimen of M. americanum from the early Rancholabrean deposits at Ziegler Reservoir in Colorado has mandibular tusks, which are often quite large (Cherney, Fisher & Rountrey, 2017; Fisher et al., 2014). Mandibular tusks are also present in the late Rancholabrean AMNH 9951. Alveoli are present in PRI 49820, but are absent in YPM12600. Green (2006) noted that mandibular tusks were always present in Irvingtonian M. americanum (n = 11) from Florida, but only present 27% of the time in Rancholabrean individuals (n = 22). An exact binomial test of goodness-of-fit comparing the M. pacificus lower tusk occurrence rate (0/13) to the Florida Rancholabrean M. americanum occurrence rate (6/22) yielded a p-value of 0.026, indicating that M. pacificus does exhibit mandibular tusks with lower frequency than M. americanum based on these data. Unfortunately, no Irvingtonian mastodon specimen from California has a preserved mandibular symphysis.

Tusks

The complete right tusk and distal third of the left tusk of WSC 18743 are preserved (Figs. 1F and 1G). Where the tusks emerge from the premaxillae they are angled ventrally about 5° relative to the maxillary tooth row, and laterally about 20° from the midline. The right tusk spirals gently both medially and dorsally, so that the tips would have pointed straight anteriorly with the neck at maximum ventral deflection, and vertically with the neck at maximum dorsal deflection. The tusks exhibit differential wear at the tip, with heavier wear on the right tusk.

Numerous other Diamond Valley Lake specimens were recovered with portions of one or both tusks, including WSC 9622, WSC 8817 (Fig. 7), WSC 10844 (Fig. 8), WSC 19730 (Fig. 9), WSC 22578 (Fig. 10), WSC 10829 (Fig. 11), WSC 10819 (Fig. 15), WSC 8904 (Fig. 16), WSC 7561, and WSC 8932. Because tusks grow continuously and can be influenced by environmental variables, evaluation of sexual dimorphism, and gender identification based on tusks must be approached with caution. That said, at least for M. americanum, at any given ontogenetic age females tend to have smaller tusks than males all (Fisher, 2008, 2009; Smith, 2010), so it is possible to make a tentative gender identification provided tusk (or alveolus) size and Laws Group can be determined. Available data suggest that in M. americanum adult females with tusk circumferences >36 cm and males with circumferences <39 cm occurred rarely if at all (Fisher, 2008, 2009; Smith, 2010). Remarkably, all of the tusks associated with maxillary teeth from Diamond Valley Lake appear to represent adult males, based on their sizes and ontogenetic ages. There are several smaller tusks, not associated with maxillary teeth that may represent either females or juvenile males, although we cannot rule out juvenile mammoths as a possibility.

Referred specimen SDSNH 86541 (Fig. 21A) appears to represent an adult female (LG XX), with very small (78 mm maximum diameter), straight tusks. While SDSNH 86541 is a particularly small individual, the tusk morphology is consistent with the larger WSC 3795 and WSC 3817. Another referred specimen, WSC 8917 (Fig. 18), is also an apparent adult female (LG XVIII or XIX). While lacking tusks, the maximum diameter of the alveolus for the left tusk is approximately 71 mm.

Dentition

Like other crown proboscideans, mammutids exhibit horizontal tooth replacement, progressing through a total of six teeth in each quadrant. Although there is a considerable amount of variation in dental morphology, in both M. pacificus and M. americanum dP2/dp2 and dP3/dp3 are bilophodont (although in both taxa dP3/dp3 often develop a posterior shelf/partial third loph, as described in Green & Hulbert (2005)), dP4/dp4, M1/m1, and M2/m2 are trilophodont. Of the 39 M. pacificus M3s examined in this study, 15 are tetralophodont and 24 are pentalophodont. All known M. pacificus m3s are pentalophodont. Summary measurements of teeth for both M. pacificus and M. americanum are listed in Tables 4–15.

WSC 18743 includes both left and right M2/m2 and M3/m3. Both the upper and lower second molars are heavily worn with only the margins of the lophs remaining. The M3/m3s show lighter but still heavy wear, especially on the anterior lophs and lophids (Figs. 24A and 24I). While the M3s of WSC 18743 are tetralophodont, more than half of known M. pacificus M3s are pentalophodont, although the fifth loph varies a great deal in size and shape (Figs. 24C and 24D). In the holotype the transverse valleys between lophs are open and the enamel is smooth with no cingulum except on the anterior and anterolabial edges of the M3s. A cingulum is also absent or only weakly developed on the M3 and m3 in all referred specimens of M. pacificus (Figs. 24B–24D and 24J–24L). The cingulum is generally poorly developed on the M3/m3 of M. americanum as well, except the anterior margin of the tooth, but there are some individuals with a strongly developed cingulum (e.g., Fig. 24F). In M. pacificus teeth anterior to M3/m3 frequently have some development of a cingulum and more rugose enamel, although the expression of these features varies across individuals. All known M. pacificus teeth are more similar in texture to the “smooth variety” teeth of M. americanum associated with spruce/deciduous forests, rather than the “rugged variety” associated with pine-parkland habitats (King & Saunders, 1984). However, “smooth variety” M. americanum teeth tend to be larger than both “rugged variety” M. americanum teeth and M. pacificus teeth (King & Saunders, 1984).

Only limited data (e.g., femoral measurements) provide body size estimates for M. pacificus, but available specimens suggest that tooth size may not correlate closely, if at all, with body size. For example, the m2 of WSC 18743 (an apparent male) has a length of 103.6 mm, while the m2 of SDSNH 86541 (Fig. 21C) (a presumed female) is only slightly longer at 104.6 mm. Yet the distal femur width of WSC 18743 is 288 mm, compared to only 203 mm in SDSNH 86541. Given the inability to determine gender from isolated teeth, comparative body size estimates between Mammut specimens should be approached with caution if based on isolated teeth.

The most remarkable feature of the dentition of M. pacificus is the narrowness of the crowns, particularly in the third molars (Figs. 24I–24P and 26; Tables 4 and 5; Table S1). The length:width (L:W) ratio of the m3 in WSC 18743 is 2.44 (left) and 2.46 (right). This pattern of narrow m3s is seen in multiple specimens of M. pacificus, with an average L:W ratio of 2.24 (N = 24; SD = 0.13; max = 2.44, min = 1.95). In contrast, the L:W ratio in M. americanum is only 1.91 (N = 121; SD = 0.11; max = 2.23, min = 1.63) (Figs. 26C and 26D; Table S2). While there is overlap in the L/W ranges between the taxa, the differences are significant (T-test value p < 0.001).

The same pattern is apparent in the M3s (Figs. 24A–24H), in which the M. pacificus average L:W ratio is 1.98 (N = 39; SD = 0.14; max = 2.33, min = 1.69) while the M. americanum average L:W ratio is 1.77 (N = 79; SD = 0.10; max = 1.95, min = 1.59). As with the m3s, these differences are significant (T-test value p < 0.001). The variation in loph count in M3 does affect L:W ratio in M. pacificus. In the 24 pentalophodont M3s L/W = 2.02, while in the 15 tetralophodont M3s L/W = 1.91 (T-test value p = 0.007). However, the frequent presence of a fifth loph alone is insufficient to explain the difference in L:W ratios between M. pacificus and M. americanum. Even when considering only tetralophodont M. pacificus M3s, M. pacificus still has significantly higher L:W ratios than M. americanum (1.91–1.77, T-test value p < 0.001).

While the L:W ratios differ between M. pacificus and M. americanum, there is little difference in the length of the teeth. The average length of m3 is nearly identical in the two taxa (183.15 mm in M. pacificus, 183.54 mm in M. americanum, Mann–Whitney test p = 0.978; Table 5). The average length of M3 is significantly greater in M. americanum (174.68–166.94 mm, Mann–Whitney test p = 0.038; Table 4), apparently due to several very large M. americanum M3s (the longest M. americanum M3 is 17.5 mm longer than the longest M. pacificus specimen, while the shortest M. americanum specimen is only 0.3 mm longer than the shortest M. pacificus specimen) (Table 4).

The pattern of narrow crowns in M. pacificus is also apparent in Irvingtonian specimens. Four Irvingtonian m3s referred to M. pacificus, including two from Idaho and two from California, have L:W ratios ranging from 2.23 to 2.38 (average = 2.32), even more narrow than the Rancholabrean average (2.24) (Table S1). Only a single Irvingtonian M. pacificus M3 has been identified, for which L/W = 2.00. Green (2006) provided measurements of a number of Irvingtonian M. americanum specimens from Florida. Four m3s had an average L/W of 1.93, and four m3s had an average L/W of 1.77, very similar to the Rancholabrean M. americanum averages of 1.91 and 1.77 (Table S2). While the data are limited, it appears that the disparity in M3/m3 proportions between M. pacificus and M. americanum was present through the entire Pleistocene.

The narrow M3/m3 crown of M. pacificus relative to M. americanum is not as obvious in other tooth positions (Figs. 24Q–24LL and 27–31; Tables 6–15), although in some cases small sample size is an issue. Even with large sample sizes, M2 and M1 of M. pacificus do not significantly differ from those of M. americanum in L:W ratio (1.32–1.29 for M2, and 1.27 to 1.23 for M1) (Figs. 27B and 28B). Nevertheless, within the data set used in this study, for 11 out of 12 tooth positions, the highest L:W ratio measured was a California tooth attributed to M. pacificus. In the lone exception, dP3, there are only two known teeth from California, and one of these had the second-highest L:W ratio.

Vertebrae

WSC 18743 includes portions of seven vertebrae (Fig. 5), including the fifth cervical, two posterior thoracic, two lumbar, and two poorly preserved specimens that are probably additional posterior thoracics. Morphologically, these vertebrae do not differ in any substantial way from corresponding elements from M. americanum, and the key features indicated by Olsen (1972) and Hodgson et al. (2008a) as distinguishing Mammut from Mammuthus are all present. One of the lumbar vertebrae (Figs. 3G and 3H) is a close morphological match for the first lumbar of PRI 8829, a somewhat larger specimen of comparable ontogenetic age (LG XXI) (Hodgson et al., 2008a). Based on comparison to PRI 8829, both thoracic vertebrae (Figs. 3C–3F) appear to be posterior to the 13th thoracic (Hodgson et al., 2008a).

Several other specimens include larger numbers of preserved vertebrae, particularly among the Diamond Valley Lake sample. WSC 7561, tentatively identified as an adult male (based on an associated tusk) includes the first 11 vertebrae in articulation (Fig. 20). The first four thoracic vertebrae have long, slender, and posteriorly directed spinous processes, increasing in height from the first to the fourth vertebra, that are expanded dorsally, a feature also seen in Mammut americanum. SDSNH 86541 (Figs. 21D–21F) and WSC 13552 are adult individuals with several associated vertebrae, all of which show complete epiphyseal fusion. These two individuals have vertebrae that are substantially smaller than those in WSC 18743 or WSC 7561; these likely represent adult females (as mentioned above, SDSNH 86541 also includes tusks that suggest this individual was a female).

Sacrum and pelvis

WSC 18743 includes a nearly complete pelvic girdle (Fig. 4). The pelvic elements are fully fused, consistent with the maturity of the specimen based on dental eruption and wear (Haynes, 1991). The sacrum consists of six fused vertebrae. A second nearly complete pelvic girdle from Diamond Valley Lake, WSC 9642 (Fig. 22), represents a younger individual than WSC 18743, with only partial fusion of the pelvic elements. Five vertebrae are fully fused in WSC 9642, with the sixth vertebra partially fused and with expanded transverse processes that are consistent with the sixth sacral of WSC 18743. A third specimen from Riverside County, SBCM 5.3.298, is an adult (LG XXI, 36 ± 2 AEY) with six fully fused sacral vertebrae. In contrast, most adult M. americanum specimens seem to have only five sacral vertebrae, including AMNH 9951, NYSM VP 58, UF 5200, and USNM 8204. PRI 8829 includes five vertebrae in the sacrum, which is fused anteriorly to the third lumbar vertebra (Hodgson et al., 2008a). There is, however, some variation in the M. americanum sacral count. According to Woodman & Branstrator (2008), CMC VP-1 from Indiana has six sacral vertebrae. Haynes (1991) mentions a specimen from the University of Wisconsin (no catalog number provided) with only four sacrals. In Mammuthus primigenius, while there are typically five sacral vertebrae, there is some individual variation (Lister & Stuart, 2010).

Green (2006), Hodgson et al. (2008a), and Fisher (2008) found that the ratio of the pelvic aperture width to the minimum width of the ilial shaft is a reliable indicator of sex in M. americanum, a relationship that was first recognized in mammoths by Lister (1996). Females have a ratio of greater than 2.6, while in males it is less than 2.6. In WSC 18743, this ratio is 2.29, indicating it is a male specimen, consistent with its large overall size and robust tusks. Likewise, WSC 9642 is also apparently a male, with a ratio of 2.45.

Femur

The only preserved limb element from WSC 18743 is the distal end of the left femur (Fig. 5). The distal femoral epiphysis is fully fused to the shaft of the femur, which is consistent with the tooth wear state and vertebral epiphyseal fusion in indicating a fully mature animal. The femur is noteworthy for its size. The maximum width across the distal end is 288 mm, far larger than any other California specimen. There are, however, several specimens of M. americanum that are larger in this dimension. For example, DMNH 1496 from Indiana has a distal width of 299 mm with a length of 1,133 mm, while DMNH 61338 from Colorado has a distal width of 323 mm, with a length of 1,180 mm.

Several other femora of M. pacificus are known, in some cases with associated skeletal elements. One particularly noteworthy example is SDSNH 86541 (Fig. 21B), which includes most of a forelimb, numerous vertebrae, both tusks, and an m2 in addition to a complete femur. The femur exhibits complete fusion of both epiphyses indicating that it is from an adult, which is consistent with the wear on m2 and indicates LG XXII. Based on the presence of a small diameter tusk combined with the ontogenetic age, this specimen is interpreted as representing a female animal. Yet this femur has a distal width of only 203 mm, with a length of just 817 mm, remarkably small for an adult mastodon. Nor is this unique among specimens of M. pacificus. LACM-HC 1266 is not associated with any dental material, but it does exhibit complete epiphyseal fusion, with a distal width of 222 mm and a length of 820 mm. The nearest comparable M. americanum femur we have examined is G25656, a specimen from Carters Bog, Ohio, of an apparent young adult female (LG XII; 18 ± 1 AEY), with a distal femoral width of 220 mm and a length of 899 mm. WSC 9622, an apparent adult male M. pacificus (LG XX) with complete femoral epiphyseal fusion has a distal femoral width of 261 mm and length of 956 mm. This is only slightly larger than PRI 49618, an apparent adult female (LG XX–XXI; Hodgson et al., 2008b; Fisher, Beld & Rountrey, 2008) with a distal width of 240 mm and a length of 950 mm (Hodgson et al., 2008a). In fact, seven California femora (Fig. 23) from five different Rancholabrean localities spread across at least 35,000 years have an average maximum length of 918.8 mm, compared to the M. americanum average of 1,041.2 mm, based on nine Rancholabrean specimens, suggesting that femora of M. pacificus may have been shorter on average than those of M. americanum. A Mann–Whitney test on the length data showed a significant difference in the medians of the samples (U-value = 10.5, p = 0.050). In a slightly smaller subsample for which data are available (six M. pacificus and eight M. americanum), femora of M. pacificus femora tend to be more robust, with a length:midshaft diameter ratio of 6.16 compared to 6.75 in M. americanum (Mann–Whitney test U-value = 1, p < 0.001) (Fig. 25).